Several degenerative retinal diseases that commonly lead to blindness, such as retinitis pigmentosa and age-related macular degeneration, are primarily caused by degradation of photoreceptors (i.e., rods and cones) within the retina, while other parts of the retina, such as bipolar cells and ganglion cells, remain largely functional.
Accordingly, an approach for treating blindness caused by such conditions that has been under investigation for some time is provision of a retinal prosthesis connected to functional parts of the retina and providing photoreceptor functionality.
Connection of a retinal prosthesis to functional parts of the retinal is typically accomplished with an array of electrodes (see, e.g., U.S. Pat. No. 4,628,933 to Michelson). Michelson teaches a regular array of bare electrodes in a “bed of nails” configuration, and also teaches a regular array of coaxial electrodes to reduce crosstalk between electrodes. Although the electrodes of Michelson can be positioned in close proximity to retinal cells to be stimulated, the electrode configurations of Michelson are not minimally invasive, and damage to functional parts of the retina may be difficult to avoid.
Alternatively, a prosthesis having electrodes can be positioned epiretinally (i.e., between the retina and the vitreous humor) without penetrating the retinal internal limiting membrane (see, e.g., U.S. Pat. No. 5,109,844 to de Juan et al.). Although the arrangement of de Juan et al. is less invasive than the approach of Michelson, the separation between the electrodes of de Juan et al. and retinal cells to be stimulated is larger than in the approach of Michelson.
Such increased separation between electrodes and cells is undesirable, since electrode crosstalk and power required to stimulate cells both increase as the separation between electrodes and cells increases. Furthermore, increased electrical power has further undesirable effects such as increased resistive heating in biological tissue and increased electrochemical activity at the electrodes.
U.S. Pat. No. 3,955,560 to Stein et al. is an example of an approach which provides low separation between electrodes and nerve fibers (i.e., axons), but requires a highly invasive procedure where a nerve is cut and then axons regenerate through a prosthesis and past electrodes embedded within the prosthesis.